It's a metaphor in which a person's perceived persistence in sobriety is represented by a steam engine. If the engine loses steam, it can't do as much work, and may be broken (or may simply need fuel, though that's not the metaphoric implication here).
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John LawlerJun 8 '13 at 0:52

John Lawler: This is an answer, not a comment on the question. It ought to be submitted as an answer, so you can get credit for it.
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Hack SawJun 8 '13 at 6:22

3 Answers
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In other words, energy, or capacity to work can dissipate and be lost.

Because of the disjunction, "or" instead of an "and" conjunction, this concern over the loss of energy may not relate directly to a loss of willpower that allows the subject to resume drinking; instead, it may refer to some other activity, like reliably showing up to work, or something else the narrator might have referred to earlier that was a condition of turning the subject's life around.

There are a number of idiomatic phrases relating to steam, the definition of which is key to understanding its use in these phrases:

steam, n.: power that is created when water is heated

The phrase lose steam (or run out of steam) means to lose energy or enthusiasm; in this context, the writer is wondering whether the homeless person being discussed will be able to sustain the energy and will necessary to keep striving toward continued sobriety and a turnaround in his life.

The saying losing steam as stated above is a metaphor but I've never heard it used specifically in reference to sobriety . It is my belief that losing steam is more a generalization as in losing motivation or becoming exhausted mentally or physically. It could certainly be used as losing motivation to stay sober but that would be redundant and since "or" is used I would not think it would be used to state the same thing. All we know of from the question and the statement is the guy is homeless and falling off the wagon usually refers to drinking/sobriety (it has been used to infer one falling back into drug abuse as well, although this is less common). If I were to make an educated guess the meaning here is more in general terms "to stay motivated" for if he's homeless it is more than likely he is without a job and a whole host of lifestyle changes. I hope I'm not coming off picky. I just think it sounds too redundant to make an assumption "losing steam" is in reference to sobriety. That's my .02

LOL talk about redundant. I did not see the answer above (my apologies for doing laundry in the middle of writing a reply) and I gave Justin an UP for not being as verbose;-)